The silicon based condenser microphone also known as an acoustic transducer has been in a research and development stage for more than 20 years. Because of its potential advantages in miniaturization, performance, reliability, environmental endurance, low cost, and mass production capability, the silicon microphone is widely recognized as the next generation product to replace the conventional electret condenser microphone (ECM) that has been widely used in communication, multimedia, consumer electronics, hearing aids, and so on. Of all the designs, the capacitive condenser microphone has advanced the most significantly in recent years. The silicon condenser microphone is typically comprised of two basic elements which are a sensing element and a pre-amplifier IC device. The sensing element is basically a variable capacitor constructed with a movable compliant diaphragm, a rigid and fixed perforated backplate, and a dielectric spacer to form an air gap between the diaphragm and backplate. The pre-amplifier IC device is basically configured with a voltage bias source (including a bias resistor) and a source follower preamplifier.
Unlike the ECM which has stored charge on either its backplate or diaphragm, the silicon microphone depends on the external bias voltage to pump the required charge into its variable capacitor. The diaphragm vibration induced by any sound signal will cause the change in capacitance as the charge is constantly maintained. The resulting voltage change is converted into a low impedance voltage output by the source follower preamplifier. In a typical ECM microphone, the diaphragm and backplate are separated by more than 10 microns and an electret bias of several hundred volts is preset by using an ion implantation process to bring the microphone sensitivity to the desired range. For a silicon microphone, the spacing between the diaphragm and backplate elements could be a few microns and an external bias voltage of about 5 to 10 volts is applied to bring the microphone to the working condition.
The success of the silicon condenser microphone is largely attributed to the fact that its structure can be embodied in various forms with most of the materials and processing techniques adopted from the semiconductor industry. The diaphragm is usually made of silicon or polysilicon although silicon nitride/metal or a composite with oxide/polysilicon/metal/polymer has also been used. Likewise, the backplate may be constructed from silicon or polysilicon with glass, nickel, polyimide/metal, or nitride/metal being alternative materials. The dielectric spacer layer that defines the air gap between the diaphragm and backplate is usually made of a nitride and/or an oxide.
However, the silicon condenser microphone has unique processing requirements that differ from semiconductor processing standards. For example, the uncertain intrinsic stress associated with the deposition process for thin semiconductor films is problematic in the sense that it can significantly affect the compliance of the silicon microphone diaphragm. If the stress is too high, the diaphragm can either buckle or become stiffened. Since the diaphragm plays a key role in determining sensitivity and frequency response performance, the diaphragm must be as compliant as possible within a given frequency range which is difficult to achieve considering the intrinsic stress variation in thin films. To address this issue, U.S. Pat. No. 5,146,435 and U.S. Pat. No. 5,452,268 to Bernstein suggested the use of a stress-free single crystal silicon diaphragm suspended with a few flexible springs. Unfortunately, the implementation of supporting springs requires some slot cuttings on the microphone diaphragm which introduces an acoustic leakage problem. The fabrication of a thin diaphragm on a silicon substrate, as suggested by the prior art, is also a very challenging task for volume production.
In U.S. Pat. No. 5,490,220 to Loeppert and PCT Patent No. WP 02/15636 to Petersen, a “free plate” concept is disclosed that allows the thin film diaphragm to be floating within certain constraints. The floating diaphragm can have its intrinsic stress relaxed after removal of sacrificial layers. However, the floating plate design requires a complex structural definition and a complicated fabrication method. Moreover, it is difficult to ascertain where the diaphragm is anchored due to the gaps between the diaphragm and constraints following the sacrificial release and drying process.
In U.S. patent application Ser. No. 2002/0106828A1, Loeppert proposes a wafer bonding method to fabricate a single crystal diaphragm with its edge supported by micro pillars. However, even a small amount of bonding induced stress may still result in an uncertainty in mechanical compliance for a thin silicon membrane. PCT patent application Ser. No. WO 01/20948 A2 discloses a CMOS MEMS diaphragm which is a composite membrane formed by patterned oxide-metal-poly layers and polymer coating and fillings. Unfortunately, it is difficult to control the strain gradient of the composite semiconductor membrane. Moreover, the polymer coating and filling makes the strain gradient issue worse because of thermal expansion mismatching.
Therefore, an improved structure of a sensing element is needed that addresses the intrinsic stress issue and simplifies the fabrication process of a silicon microphone.